Method for cooling and spheroidizing steel rod

ABSTRACT

SPHEROIDIZED STEEL ROD SUITABLE FOR COLD WORKING IS OBTAINED BY HOT ROLLING COOLING AND REHEATING STEEL ROD IN DIRECT SEQUENCE. COOLING THROUGH TRANSFORMATION IS DONE IMMEDIATELY AFTER HOT ROLLING IN ORDER TO INHIBIT THE FULL GROWTH OF THE SMALL AUSTENITE GRAINS WHICH RESULT FROM ROLLING, PRIOR TO TRANSFORMATION. THE COOLING THROUGH TRANSFORMATION IS RAPID ENOUGH TO FORM CONSTITUENTS SUCH AS FINE PEARLITE, BAINITE AND MARTENSITE IN SUBSTANTIAL AMOUNTS TO RENDER THE TRANSFORMED ROD TOO HARD OR BRITTLE FOR THE INTENDED COLD WORKING AND TO SUPRESS SUBSTANTIALLY THE DEVELOPMENT OF PROEUTECTOID FERRITE. DIRECTLY THEREAFTER, BEFORE SAID ROD HAS COOLED TO AMBIENT TEMPERATURE, THE ROD IS PASSED THROUGH A SPHEROIDIZATION FURNACE, WHILE THE ROD IS IN THE FORM OF OVERLAPPING NON-CONCENTRIC RINGS, TO HEAT THE ROD TO A TEMPERATURE OF ABOUT 1280*1330*F. (690-720*C.) AT LEAST UNTIL THE CEMENTITE IN THE MICROSTRUCTURE BEGINS TO COALESCE INTO SPHEROIDAL PARTICLES. IN ONE EMBODIMENT THE ROD IS THEN COOLED AND STORED FOR FURTHER TREATMENT. IN ANOTHER EMBODIMENT IT IS MAINTAINED AT THE TEMPERATURE OF TRETMENT UNTIL ENLARGEMENT OF THE SPHEROIDAL PARTICLES OCCURS. THE PROCESS IS GENERALLY APPLICABLE TO PLAIN CARBON AND ALLOY STEEL ROD WHICH, FOR ONE REASON OR ANOTHER, IS NOT SUITABLE WITHOUT SPHEROIDIZING FOR CERTAIN COLD-WORKING PROCEDURES, AND IT PROVIDES A NOVEL METHOD OF SPHEROIDIZING TO OBTAIN RAPIDLY A UNIFORMLY ANNEALED PRODUCT.

Jan; 16, 1973 v. J. VITELLI 3,711,333

METHOD FOR 000mm AND SPHEROIDIZING STEEL non Filed Oct. 16 1970 4 Sheets-Sheet 1 FIG.2

INVENTOR. VITO J. VITELLI BY /149w;

ATTORNEYS Jan. 16, 1973 v. J. VITELLI 3,711,333

METHOD FQR COOLING AND SPHEROIDIZING STEEL ROD Filed Oct. 16. 1970 4 Sheets-Sheet 2 l7 I? E g.

I I F|G.3

FIG.4

INVENTOR.

VITO J. VITELLI AT TORNEYS Jin. 16, 1973 v..:. VITELLI 3,711,338v

METHOD FOR COOLING AND SPHEROIDIZING STEEL ROD Filed Oct. 16. 1.970 4 Sheets-Sheet 4 United States Patent O 3,711,338 METHOD FOR COOLING AN D SPHEROIDIZING STEEL ROD Vito J. Vitelli, Shrewsbury, Mass, assignor to Morgan Construction Company, Worcester County, Mass. Filed Oct. 16, 1970, Ser. No. 81,271 Int. Cl. C21d 7/14 US. Cl. 14812 9 Claims ABSTRACT OF THE DISCLOSURE Spheroidized steel rod suitable for cold working is obtained by hot rolling, cooling and reheating steel rod in direct sequence. Cooling through transformation is done immediately after hot rolling, in order to inhibit the full growth of the small austenite grains, which result from rolling, prior to transformation. The cooling through transformation is rapid enough to form constituents such as fine pearlite, bainite and martensite in substantial amounts to render the transformed rod too hard or brittle for the intended cold working, and to suppress substantially the development of proeutectoid ferrite. Directly thereafter, before said rod has cooled to ambient temperature, the rod is passed through a spheroidization furnace, while the rod is in the form of overlapping non-concentric rings, to heat the rod to a temperature of about l 280 1330" F. (690720 C.) at least until the cementite in the microstructure begins to coalesce into spheroidal particles. In one embodiment the rod is then cooled and stored for further treatment. In another embodiment it is maintained at the temperature of treatment until enlargement of the spheroidal particles occurs. The process is generally applicable to plain carbon and alloy steel rod which, for one reason or another, is not suitable without spheroidizing for certain cold-working procedures, and it provides a novel method of spheroidizing to obtain rapidly a uniformly annealed product.

BACKGROUND OF THE INVENTION This invention relates to the annealing of steel rod, both plain carbon and alloy steel rod, which after controlled cooling is too hard or brittle for its intended coldworking. The invention relates to a process of controlled cooling and in direct sequence spheroidizing steel rod to improve its physical properties, by a novel method which permits such treatment to be accomplished rapidly and efiiciently.

spheroidizing is a heat-treating step, normally applied to completely coiled rod coils in group or batch fashion, that converts the cementite of the steel into spheroidal form, by heating, customarily for an extended period of approximately one hour or more. The spheroidizing temperature must ordinarily be kept below the lower critical transformation temperature, in order to avoid the formation of austenite. Thus, the treating temperature is usual- 1y about 12501330 F. (675-720 although this may be exceeded for very low carbon and hypereutectoid steels, for which treatment at temperatures up to 1350 1435 F. (730-780 C.) or higher, may be desirable.

The microstructure of certain hot rolled rod cooled through transformation may make it unsuitable for a specific use, such as wire drawing, or a limited amount of wire drawing followed by a cold heading operation, and in such cases spheroidization or equivalent annealing is required in order to prepare the rod for processing. Spheroidization can alter the physcal properties of the treated rod favorably, by rendering it softer and more malleable. Spheroidized steel rod, consequently, is more readily cold worked and has greater ductility.

3,711,338 Patented Jan. 16, 1973 For most steel rod, however, spheroidizing takes place only after extended time at the elevated temperature. It is impossible to accelerate the process by heating at higher temperature because of the risk of austenizing the product. Even under ideal temperature control the process takes 5-10 hours at 1300-1330 F. (700720 C.), or even longer, for typical low-carbon steel. The process is complicated of course if the temperature control is less than ideal, and particularly if the rod configuration makes uniform heating difiicult. Under these typical conditions, it is not uncommon to provide a margin of safety, by spheroidizing at somewhat lower temperatures, e.g. about 1250 F. (675 C.), which of course extends the treatment period even farther, to 15-20 hours or perhaps longer. Further, if the microstructure of the steel rod is not uniform throughout its length, and as a consequence the period required for spheroidization is not of equal duration throughout, it is necessary to treat the entire product for as long as it takes to treat that part which requires spheroidizing for the longest time. All of these factors make the process of spheroidizing a cumbersome step, requiring careful heat treatment at elevated temperature for extended periods.

It is a major objective of this invention to provide a method of spheroidizing steel rod which at once both minimizes the treatment period and provides a high degree of temperature control. Other objectives and advantages of the invention will become apparent as it is described in detail.

BRIEF DESCRIPTION OF THE INVENTION The present invention is directed to an improved spheroidization process in which, broadly, steel rod is hot rolled and immediately thereafter, in order to inhibit the full growth of austenite grains, is cooled through transformation rapidly enough to suppress the development of proeutectoid ferrite in the transformed rod and to form constituents such as fine pearlite, bainite and martensite which render the transformed rod too hard or brittle for it intended cold working, and then is passed through a treating furnace at a temperature of about 12502000 F. (675-1100 C.) until the cementite of the rod is spheroidized sufficiently to render it sufficiently ductile for its intended cold working. During both the controlled cooling and the spheroidization the rod is in the form of overlapping, non-concentric rings, and due to the high heattransfer efliciency of its configuration, the rod is rapidly cooled and rapidly brought to a uniform spheroidizing temperature in the range of 12501330 F. (675720 C.), and preferably for a brief period to a higher temperature without reformation of austenite. Thus, spheroidization is achieved rapidly and uniformly throughout the rod length.

Preferably, the spheroidization step is performed immediately after the rod has been passed through transformation by controlled cooling while the rod is in the form of overlapping, non-concentric rings, after which the rod may be passed into a spheroidizing furnace.

BRIEF DESCRIPTION OF THE DRAWINGS AND PHOTOMICROGRAPHS The present process will be better understood by reference to the drawings which illustrate apparatus suitable for carrying out the method. In the drawings:

FIG. 1 is an elevation view of apparatus for the controlled cooling of steel rod through transformation directly in line with a rolling mill;

FIG. 2 is an elevation view of a roller-hearth furnace, which in this instance is in sequence directly beyond the apparatus of FIG. 1, for spheroidizing the rod;

FIG. 3 is an enlarged cross-sectional view, taken along line 33 of FIG. 1, showing the inside part of the controlled-cooling apparatus; and

FIG. 4 is an enlarged cross-sectional view, taken along line 4-4- of FIG. 2, showing the inside part of the rollerhearth furnace during spheroidization.

FIG. 5 is a series of four photomicrographs showing the microstructure of the following rods: A, rod after rapid cooling through transformation, essentially completely martensite; B the same rod after spheroidization; C, another rod after rapid cooling through transformation, essentially a mixture of bainite and martensite; and D, the same rod after spheroidization. I

FIG. 6 is a series of four photomicrographs showing various rods after spheroidization: E, spheroidized fine, unresolvable pearlite; F, a mixture of coarse pearlite and spheroidized bainite or martensite; G, spheroidized fine, unresolvable pearlite; and H, slightly spheroidized bainite.

DETAILED DESCRIPTION OF THE INVENTION In accordance with the present invention, first, steel rod is hot rolled and formed into overlapping, non-concentric rings upon a conveyor, and immediately subjected to a rapid, controlled cooling through transformation. Because the austenite grains of the hot-rolled steel are very fine and are not permitted to grow substantially because of immediate transformation, and because of the ring configuration, the transformation is accomplished uniformly and rapidly. The transformed rod contains constituents such as fine pearlite, bainite or martensite, which render it unsuitable for at least some cold-working procedures. Then, as a second step, preferably in direct sequence with the cooling step and in the same overlapping ring form upon the conveyor, the rod is subjected to spheroidization in a furnace at about 1250-2000 F. (675-1100 (3.), whereby the rod reaches a temperature of about 1250-1330 F. (675720 C.) until the cementite in said constituents spheroidizes to form a rod product suitable for the intended cold working.

With reference to the figures, steel rod 10 emerging from the last stand of a rolling mill, not shown, and watercooled to a temperature of about 1200-1500 F. (650 815 (1.), is fed into laying head 11, and formed into overlapping rings 12., which are laid onto conveyor 13. Conveyor 13 is a continuous type including drive chains 14 and drive motor 15. A roller conveyor would be equivalent for the purposes of the invention. The chains have upwardly extending teeth 16 which serve to engage successively the over-lapping rings and carry them along the conveyor. The over-lapping rod rings pass along the conveyor supported by guide bars 17 into and through hood 18, as a blast of cooling fluid, such as air, is passed upwardly in contact with the rod rings by blowers 19, as shown by the arrows in FIG. 3.

The cooling medium preferably passes the rod as it is conveyed through the apparatus roughly in proportion to the mass flow of the rod, in order that the cooling may be as uniform as possible. Thus, the cooling medium is provided in greater volumetric rate at those locations, i.e., the sides of the conveyor, where the greater mass of rod is passed. This effect may be achieved 'by providing slots under and across the conveyors of varying width, such that the amount of cooling medium passed upwardly in contact with the rods is proportioned to the mass flow thereof at any point across the rings. Further, means not shown are preferably provided to vary the total rate of flow of cooling medium-in addition to the means providing proportional flow across the conveyorin order to provide for either variations in rod delivery rates or to alter the cooling rate of the rod for metallurgical purposes.

The apparatus and procedures described above have come to be associated with a method known as controlled cooling of steel rod. This method, described and claimed in several patents, e.g., U.S. Nos. 3,231,432 and 3,390,-

871, both incorporated herein by reference, results in a. rod having uniform physical properties from end-to-end of a bundle without the necessity of further heat treatment, a reduction of scale loss, and a proportioned reduction of magnetite in the scale, among numerous other advantages. For purposes of the instant invention, the controlled cooling process makes it possible, both for carbon and alloy steels, to provide a uniform rod which can undergo spheroidization in a minimum period of time, in order to obtain a product having just the properties desired for subsequent processing. The rod product of the controlled cooling process is described in detail and claimed in the U.S. Pat. No. 3,320,101, incorporated herein by reference.

Controlled-cooled rod of medium to high carbon steel is ordinarily suitable without additional processing for subsequent cold working, such as wire drawing, due to its excellent physical properties, as above described. For certain types of low, or low-to-medium carbon steel, however, and particularly for alloy steel rod, rapid, controlled cooling does not always yield a processed rod suitable for cold working, such as wire drawing, cold heading, coining, or some other form, without further annealing. Rods containing significant proportions of alloying elements such as chromium, nickel, and molybdenum, either as intentional constituents or as residual elements re maining from the steel making process, may contain martensite after rapid controlled cooiing, which is detrimental to any subsequent cold working. The present process offers a way of obtaining for all types of steel rod both the benefits of uniformity and rapid cooling that result from controlled cooling, and at the same time a product which is suitable, without further processing, for cold working. These benefits are attainable by the present method of spheroidizing the rod while in the form of overlapping, non-concentric rings, preferably immediately in line after a controlled-cooling step.

In any case, the rod to be spheroidized is carbon or alloy steel rod that has been subjected to a rapid cooling through transformation immediately after hot rolling, as described above. The normal rapid cooling rates through transformation produce suitable microstructures in plain high carbon steels which are amenable to extensive cold working in wire drawing. If the carbon steel rods are intended for such operations as cold heading or cold forging, the transformation products so produced may be too fine and the rod might be too hard. The successful cold working of alloy steel rods would be precluded if any martensite or extensive amounts of bainite were contained in the microstructure. With the rapid cooling through transformation immediately after hot rolling, as generally described above, martensite and bainite formation would be expected in alloy steel rods. Thus, the rod to be spheroidized by the present method is preferably the transformation product of relatively fine-grained austenite, containing constituents such as fine pearlite, bainite or martensite rendering it unsuitable for at least some cold working. In plain carbon steels this type of rod is relatively free of proeutectoid ferrite. On the other hand, the rapid cooling may produce a plain carbon steel rod too hard for its intended end use. With alloy steel rods, the rapid cooling would produce martensite and bainite.

The present process has significant utility for alloy steel, i.e. steel containing significant quantities of alloying elements added to effect change to its mechanical or physical properties. Under the very rapid transformation of the controlled cooling process employed here, alloy steels often contain bainite or martensite sufiicient to render the alloy rod unsuitable for cold working without further treatment. It is advantageous, nevertheless, to transform such alloys as rapidly as possible in accordance with the present process, because thebainite and martensite, and in particular martensite, tend to spheroidize more rapidly than the pearlitic structures, which would be favored by less rapid transformation. Even plain carbon steel suitable for wire drawing may be unsuitable for cold working such as cold heading, because of fine pearlite in its microstructure-or bainite or martensite, which would render it unsuitable for cold heading, because of fine pearlite in its microstructure-or bainite or martensite, which would render it unsuitable for cold working of virtually any type. The best approach, then, is totransform the rod, whether plain carbon or alloy, as rapidly as possible in order to favor the transformation constituents which are more readily spheroidized, namely, in descending order, martensite, bainite and fine pearlite, notwithstanding that these constituents are ordinarily undesirable, in the same order, since they do or could render the rod unsuitable for cold working.

In certain cases, the present process may be employed where the steel rod is either slightly water cooled, or not water-cooled, before being subjected to the rapid cooling step. This procedure would be desirable where some austenite grain growth prior to cooling would increase hardenability, thus enhancing the likelihood of martensite or bainite formation.

In one embodiment of the present process, alloy steel rod is cooled through transformation as rapidly as possible in ambient forced air, in order to promote the formation of martensite, as a major constituent of the transformed rod in order to render it more amenable to spheroidizing. By the term, major constituent of martensite, the intention is to obtain a product having a microstructure appearing in conventional photomicrographs, such as presented here, to have at least a major area of martensite. Alloys suitable for the present process include, for example, those listed at pages 61-62 of Metals Handbook, vol. 1 (8th ed. 1961), and the free-cutting carbon steels listed on page 62. By controlled cooling with forced air as rapidly as possible, in accordance with one embodiment of the present process, most of said listed alloys should yield such a martensitic structure.

Again with reference to the figures, the rod 10 is transported from the controlled-cooling apparatus (e.g., FIGS. 1 and 3), onto rollers 20, again in the form of overlapping non-concentric rings 12. Rollers 20 driven by roller drive 21 convey the rod rings through roller hearth furnace 22, where the temperature of the rods is rapidly raised to the spheroidizing point, in the range l2501330 F. (675- 720 C.). Briefly, toward the end of the furnace run, the rod may tolerably exceed the ideal spheroidizing temperature, to a temperature of up to 1350 F. (730 C.), since in the present process temperature control is so readily maintained. In any event, the reformation of austenite should be avoided. Preferably, the furnace is maintained at about 1500-2000 F. (8151l00 C.), although lower temperatures, of about 1250-1500 F. (675-815 C.) may be required if extended periods of time prove necessary. The furnace is heated by elements 23, which may be gas fired heaters or any other suitable type. Preferably, means are provided to direct a blast of heating medium over the rod in order to increase the heat transfer rate and reduce the time necessary to bring the rod to spheroidizing temperature. If it is desired to maintain further scale formation at a minimum, an inert furnace atmosphere such as nitrogen, or even the products of combustion if a direct fired furnace is used, would be preferred.

Upon leaving the spheroidizing furnace, the rod cools slowly to a point where no further metallurgical changes will occur, and the rod is suitable for handling, and collected by coiling means 24, or by other suitable procedures. Alternatively, the rod may be cooled, by means not shown, rapidly upon leaving the spheroidizing furnace to a temperature where no further metallurgical changes will occur and the rod is suitable for handling. As a further alternative, the rod, with or without intermediate cooling, may be conveyed directly to other processing stations, for example, for descaling.

The above apparatus is not intended to be necessarily the only type suitable for carrying out the present process, which is broadly to submit transformed steel rod, either low, medium or high carbon steel or alloy steel, having constituents rendering it unsuitable for cold working, to spheroidizing temperatures while in the form of overlapping non-concentric rings, in order to render it suitable for cold working. For example, it is not essential, although perhaps desirable, to have separate cooling apparatus and spheroidizing apparatus, each with its own conveyor. It may be preferable to have a single conveyor which conveys the rod directly through both the cooling zone and the annealing zone. Also, it may be desirable to have a portable spheroidizing furnace on rollers adjacent the conveyor after the controlled cooling section, which can be moved into place whenever rod products requiring spheroidization are being controlled cooled, Such an arrangement would permit changeover to or from the present spheroidization process in line with conventional c0ntrolled cooling apparatus, with little or no delay time.

Among the advantages of this process are uniformity and rapidity in spheroidizing both due in part to the overlapping pattern of the rod and to the uniformity of the transformed starting material. It is advantageous in the process to carry out the spheroidizing in direct sequence with controlled cooling, so that a uniform rod in overlapping ring form may be delivered directly to the spheroidizing apparatus.

When plain carbon steel rods are subjected to controlled cooling in line and immediately after hot rolling, the fine austenite grains which result from hot rolling are inhibited in their growth. In other words, the austenite grain size at transformation is, as a result of the rapid cooling, substantially finer than would result if the rod had been cooled in open air after hot rollingand this is the result of inhibiting austenite grain growth. Further, the cooling through transformation is rapid enough to inhibit the development of proeutectoid ferrite in the microstructure, again as compared with that which would develop by open air cooling after hot rolling. When the same process of controlled cooling is applied to alloy steel rods, the fine autenite grains which result from hot rolling, also, are inhibited in their growth. Further, the cooling through transformation is rapid enough to cause the formation of martensite or bainite. In the case of plain carbon steels, the more rapid the cooling through transformation, the finer the lamellar cementite. In the case of alloy steels, the more rapid the cooling rate through transformation, the more martensite and bainite in the microstructure. Thus, the rod to be spheroidized by this method is preferably the transformation product of relatively fine-grained austenite. The microstructures would consist of one or more of the following constituents: ferrite, pearlite, fine pearlite, bainite and martensite. Preferably, the microstructure should have the carbide and the carbide forming elements as finely dispersed as possible. In any event, the product to be spheroidized has a microstructure characterized by fine pearlite, bainite or martensite, the finely dispersed carbides of which are spheroidized in accordance with the present process.

The following examples will demonstrate the application of the present invention to the spheroidization of steel rods of various composition, which due to their microstructure after transformation are not ideally suited for certain cold working procedures, but which having undergone spheroidization acquire the desired properties.

EXAMPLE 1 FIG. 5A shows the microstructure of a %2-lI1Cl1 diameter 52100 grade hot rolled rod that had been heated in a muffle tube furnace at 1800 F. for five minutes, completely austenitized, and then air blast cooled at a rate of approximately 13 F. per second, this fast cooling rate producing an essentially completely martensitic microstructure.

FIG. B shows the microstructure of a -inch diameter 52100 grade hot rolled rod, obtained from the same rod coil adjacent to the sample shown in FIG. 5A, that had been heated in the same mufile tube furnace at 1800 F. for five minutes, completely austenitized, air blast cooled at a rate of approximately 13 F. per second down to a temperature of approximately 300 F., and then reheated to approximately 1425 F. for a period of approximately 1 minute, the furnace being at a temperature of 1800 F., removed from the furnace and allowed to cool in natural convection air at a rate of approximately 5 F. per second. It should be noted that the microstructure of FIG. 53 contains pearlite, as well as spheroidized bainite or martensite, and in this respect is significantly different from the microstructure of FIG. 5A, which was substantially, entirely martensite, even though both are the same steel and were cooled rapidly under the identical conditions.

EXAMPLE 2 FIG. 5C shows the microstructure of a .4-inch diameter 9254 grade hot rolled rod that had been heated in a mufile tube furnace at 1800 F. for 4.5 minutes, completely austenitized and then air blast cooled at a rate of approximately 14 F. per second, this fast cooling rate producing essentially a mixture of bainite and martensite.

FIG. 5D shows the microstructure of a A-inch diameter 9254 grade hot rolled rod, the sample being obtained from the same rod coil and adjacent to the sample of FIG. 5C, that had been heated in the same muflie tube furnace at 1800 F. for 4.5 minutes, completely austenitized, air blast cooled at a rate of approximately 14 F. per second, down to a temperature of approximately 300 F., and then reheated to approximately 1400 F. for a period of approximately one minute, the furnace being at a temperature of 1800 F., removed from the furnace and allowed to cool in natural convection air at a rate of approximately 6 F. per second. A spheroidized microstructure can be observed.

EXAMPLE 3 FIG. 6E shows the microstructure of a -inch diameter 1065 grade hot rolled rod, that had been heated in the same mufiie tube furnace at 1800 F. for 12 minutes, completely austenized, cooled in natural convention air to ambinent temperature at a rate of approximately 5 F. per second, reheated to 1350 F. for one minute, cooled in natural convection air at a rate of about 5 F. per second. The microstructure shows spheroidized fine, unresolvable pearlite.

EXAMPLE 4 FIG. 6F shows the microstructure of a A -inch diameter 52100 grade hot rolled rod that had been heated in a muifie tube furnace at 1800 F. for 5.5 minutes, com pletely austenitized, and then air blast cooled at a' rate of about 12 F. per second to 300 F., reheated to 1350 F. for one minute, and air cooled at a rate of about 5 F. per minute. The microstructure shows a mixture of coarse pearlite and spheroidized bainite or martensite.

EXAMPLE 5 FIG. 6G shows the microstructure of a -inch diameter 1065 grade hot rolled rod, that had been heated in a muffle tube furnace at 1800 F. for 12 minutes, completely austenitized, and then air blast cooled at a rate of approximately 12 F. per second down to a temperature of approximately 900 F., and then reheated to approximately 1350 F. for a period of about one minute, removed from the furnace and cooled in natural convection air at a rate of about 5 F. per second. The microstructure shows spheroidized fine, unresolvable pearlite;

. 8 EXAMPLE 6 FIG. 6H shows the microstructure of a Ai-inch diameter 9254 grade hot rolled rod, that had been heated in the same muflle tube furnace at 1800 F. for 4.5 minutes, completely austenitized, air blast cooled at a rate of approximately 14 F. per second to about 400 F., and then reheated to approximately 1350 F. for a period of approximately one minute, in a furnace at 1800 F., removed from the furnace and allowed to cool in natural convection air at a rate of approximately 6 F. per second. The microstructure shows slightly spheroidized bainite.

The heat treatment method described herein would be applicable to any plain carbon or alloy steel requiring :spheroidization prior to cold working. Listed below are some typical alloy steels that would fall into this category, along with the tensile strength of the steels after having been subjected to conventional controlled cooling in line with the rod mill and the tensile strength of the steels after they had been spheroidized in accordance with this process.

TENSILE STRENGTH Normal controlled cooling procedures directly in line with a rod rolling mill have cooling rates through transformation varying from 10 to 20 F. per second. These rates are sufiiciently fast so that the microstructures of the above alloy steel grades obtained under the aforementioned conditions consist of a mixture of ferrite, some resolvable pearlite, fine unresolvable pearlite, martensite, and bainite. After in line spheroidizing, the tensile strengths have been reduced, and the undesirable hard microstructural constituents, such as fine pearlite, bainite and martensite have started to spheroidize. This spheroidization results in a lowering of the tensile strength of the rod, and an increase of the ductility of the rod. The lowering of the tensile strength and the increase in ductility are the result of the spheroidizing or coalescing of the carbides in the undesirable hard microconstituents.

Each of the various plain carbon and alloy steels would require its own unique time temperature heat treatment cycle in order to bring about optimum spheroidizing conditions. These conditions can be readily determined by skilled metallurgists. Times can be varied by adjusting conveyor speeds from 20 seconds to as much as five minutes or more. Furnace temperatures would vary from approximately 1250 F. (675 C.) to as high as 2000 F. (ll00 (3.). The tonnage rates processed, mass flow rate, would of necessity be the same as that being processed continuously on the rod mill.

I claim: j 1. A method of cooling steel rod rapidly through transformation and spheroidizing the rod in order to render it more suitable for cold working, which comprises:

hot rolling and immediately cooling a length of steel rod through transformation at a rate of about 10 to 20 F. per second, to obtain a product having substantially uniform and physical properties throughout its length, and having a microstructure of fine pearlite, bainite or martensite constituents rendering the transformed rod unsuitable for its intended cold working, and said cooling being sufiiciently rapid to inhibit the growth of austenite grains prior to transformation and to inhibit the development of proeutectoid ferrite upon transformation;

placing the rod product on conveying means in the form of overlapping, non-concentric rings and conveying said rod in such form through an annealing furnace at a temperature of about 12502000 F. for a period, between 20 seconds and five minutes, sufficient at least to initiate spheroidization of said constituents and to render the rod suitable for its intended cold working, but insufficient to result in substantial conversion to austenite; and

removing said rod from the furnace and collecting the rod. 2. The method of claim 1, wherein the temperature of said annealing furnace is about l500-2000 F.

3. The method of claim 1, wherein said steel rod is an alloy steel, and wherein said cooling is done in the presence of forced air substantially as rapidly as possible, in order to promote transformation to martensite, whereby said martensite is more readily spheroidized than fine pearlite or bainite.

4. The method of claim 1 wherein said steel rod is heated in the annealing furnace to a temperature between about l300-1350 F. for a brief period insuflicient to cause substantial transformation to austenite.

5. A method of treating plain carbon or alloy steel to form steel rod suitable for cold Working, which comprises: heating a billet of plain carbon or alloy steel to a temperature in excess of its austenite transformation temperature and for a period sufficient to convert substantially all of the steel to austenite form;

rolling said billet in a rolling mill at a temperature above the transformation temperature to produce a microstructure characterized by fine-grained austenite;

immediately depositing the rolled rod in the form of overlapping, non-concentric rings on a conveyor and conveying the rod rings through a cooling zone while cooling the rod through transformation at a rate of about to F. per second, to form a product characterized throughout its length by substantially uniform physical properties and microstructure containing fine pearlite, bainite or martensite constituents rendering the transformed rod unsuitable for its intended cold working, said cooling being sufiiciently rapid to inhibit the growth of the fine-grained austenite prior to transformation and to inhibit the development of proeutectoid ferrite upon transformation;

conveying said steel rod, after transformation is sub stantially complete, still in the form of overlapping, non-concentric rings, through an annealing furnace at a temperature of about 1250-2000 F. while heating the steel rod rapidly to a temperature of about 1250-1330 F. for a period, between 20 seconds and five minutes, sufficient at least to initiate spheroidization of said constituents, and to render the transformed rod suitable for its intended cold working, but insufiicient to result in substantial conversion to austenite; and

removing said steel rod from the furnace and collecting the rod.

6. The method of claim 5, wherein the temperature of said annealing furnace is about 1500-2000 F.

7. The method of claim 5, wherein said steel rod is selected from the group consisting of alloy steels and wherein said cooling is done in the presence of forced air as rapidly as possible, in order to promote transformation to martensite and to render the transformed product more amenable to spheroidization.

8. A method of treating alloy steel to obtain a uniform alloy rod product suitable for cold working, which comprises:

heating a billet of alloy steel to a temperature in excess of its austenite transformation temperature and for a period sufficient to convert substantially all of the steel to austenite form;

rolling said billet in a rolling mill at a temperature above the transformation temperature to produce a microstructure characterized by fine-grained austenite;

immediately depositing the rolled rod in the form of overlapping, non-concentric rings on a conveyor and conveying the rod rings through a cooling zone while cooling the rod through transformation at a rate of about 10 to 20 F. per second, to form a product characterized throughout its length by substantially uniform physical properties and microstructure, said cooling being by means of forced ambient air as rapidly as possible in order to promote transformation to martensite, as a major constituent of said transformed rod, rendering it unsuitable for its intended cold working;

conveying said steel rod, after transformation is substantially complete, still in the form of overlapping, non-concentric rings, through an annealing furnace at a temperature of about 1250 2000 F. while heating the steel rod rapidly to a temperature of about 1250-1330 F. for a period, between 20 seconds and five minutes, sufficient at least to initiate spheroidization of said major martensite constituents, and to render the transformed rod suitable for its intended cold working, but insufficient to result in substantial conversion to austenite; and

removing said alloy steel rod from the furnace and collecting the rod.

9. The method of claim 8, wherein the temperature of said annealing furnace is about 1500-2000" F.

References Cited UNITED STATES PATENTS 2,938,820 5/1960 Turner l48l34 3,390,871 7/1968 McLean et al l48l56 3,506,468 4/1970 Geipel et al l48l2.4

WAYLAND W. STALLARD, Primary Examiner US. Cl. X.R. 14812.4 

